Here's a problem I had, and from looking around other people might have had it over the last year or so.

On occasion (somewhat regularly over the last year or so), if I unplugged my laptop and subsequently plugged it back in, my filesystem would be remounted read only. I was unable to remount it, with mount complaining that /dev/sda1 was unmountable (forget the exact phrase). This is on a thinkpad t61p (from 2007). On a reboot, everything would work. Looks like laptop-mode was doing something that hurt my setup.

Looking through the logs, I noticed laptop-mode had a few complaints (edited for brevity):

Most people suggested the remount and unremountable nature of /dev/sda1 was a sign of hardware failure (which could be true). Lots of posts about resetting ata1 also suggested kernel bugs or hardware failure.

The failure/remounting read-only only happened when changing power sources, so I initially thought the hardware was unable to cope with the power changes or something. I checked hdparm's settings after the failure (and before rebooting), and everything seemed reasonable (no low power states, no big warnings). I tried hdparm's device reset and a few other things, and nothing worked.

The laptop-mode complaints were a bit suspicious, so after I rebooted, I uninstalled laptop-mode and haven't had any issues since. ata1 doesn't get hard reset any more, and the root fs isn't remounted read only. My hunch is that laptop-mode was issuing some command that forced the reset/remount, perhaps due to my bad configuration.

Hope this helps. If the problem occurs again for me, I'll reply to this post.